But Mr. Leach's probe turned up blinking yellow lights. He found that such a conversion – which would boost his taxable income in the year he completed it – could scotch his eligibility for a tuition tax credit on his son's college costs. Taxes posed another quandary: "Did I want to commit to the current rates when I expect to be in a lower tax bracket when I retire?" asks Leach, a finance professor at the business school of the University of Colorado at Boulder.

His new thinking: "If I do a Roth IRA conversion [from a 403(b) account still in a former employer's retirement plan] it will only be a modest one," he says. "A Roth IRA conversion this year isn't the 'opportunity of a lifetime' people might have thought from all the hoopla about it."

Six months into the start of the expanded opportunity to convert to a Roth IRA, Americans aren't exactly rushing to sign up. Only 14 percent of respondents were considering switching standard IRA assets to a Roth IRA this year or next, according to an April survey by Boston-based Putnam Investments.

Some 72 percent of baby boomers who own an IRA told San Antonio-based USAA, a financial-services company, that they weren't planning to switch their standard IRA to a Roth version this year.

To be sure, many people are drawn to the attractions of a Roth conversion – among them, the ability to pay taxes on the converted amount at today's rates, after which the Roth account's assets grow free of federal income taxes. Moreover, unlike investors in standard IRAs, who must begin taking distributions at age 70-1/2, Roth IRA holders never need to take distributions, which can leave them with more money in the accounts to bequeath to heirs, if they choose.

And if they act this year, converters to Roth IRAs can choose to pay the taxes due on the converted sums this tax year, or split them between the 2011 and 2012 tax years.

Such attractions have stirred some action. During this year's first five months, Fidelity Investments logged about four times as many conversions as during the same period last year. T. Rowe-Price, Vanguard, and Schwab have also seen spikes in interest. "And we expect perhaps a bigger surge at year's end," said Casey Mervine, a financial consultant in Schwab's Torrance, Calif., office.

Roth conversions best suit the wealthy, says Jeffrey Levy, president of Harland Financial Services in New York City. "Their primary concern is taxes," he says, and over their lifetimes they'd expect that asset growth in their Roth accounts would "dwarf the taxes they'd pay today" for the conversion.

For most people, the conversion opportunity is "not that big an issue," Mr. Levy adds. People with modified adjusted gross incomes below $100,000 could already make the conversion before this year. But many of these people probably lack the funds outside their retirement assets to pay the taxes incurred, he says.

Tax rates and other variables can also muddy the conversion picture. While experts say a Roth conversion sounds attractive, further study may churn up complications. For instance, on the tax front, if people plan to take IRA withdrawals when retired, but expect to be in a lower tax bracket, then they won't find a Roth conversion particularly attractive, says Richard Kaplan, law professor at the University of Illinois.

Moreover, Roth IRA conversions can have additional costs that aren't obvious, says Professor Kaplan. For example, amounts converted could temporarily affect how much of one's Social Security benefits are taxed. They could also alter some Medicare Part B beneficiaries' premiums and hinder their ability to take tax deductions for medical costs.

In addition, since many states base their state income taxes on a taxpayer's federal taxable income, a conversion to a Roth IRA could boost some converters' state tax bill, he says.

The IRA conversion process "is more complicated than a lot of people have made it seem," Kaplan adds. Before people take the plunge, he says, "[they] need to crunch some serious numbers."